I'm hoping that we can remove the TeX code from beginproblem() and place it in the global tex header files so that these errors are less mysterious. Maybe release 2.11?
I believe that it also has something to do with the fact that you were using PGML. There is some tex code in the beginproblem() macro related to PGML typesetting which makes the order in which one inserts things like beginproblem() and $BEGIN_ONE_COLUMN delicate.